This invention is related to ignition modules for internal combustion engines. In particular, it is an electronic module that creates a timed open circuit to generate the high voltage necessary to fire spark plugs in gasoline engines for lawn mowers, chain saws and the like that have magneto ignition systems and do not have batteries to supply the energy for ignition.
The production of a voltage of the order of kilovolts that is necessary to jump the gap in a spark plug is typically effected by the use of an induction coil in which a relatively high current in a primary winding is interrupted, producing a relatively high L di/dt voltage. This voltage is stepped up by transformer action in the induction coil to produce the sparking voltage. In small engines that do not have batteries, it is common to obtain the current that is interrupted by induction from a permanent magnet that is rotated in synchronism with the engine. This is often referred to as a magneto ignition. Rotation of the permanent magnet so as to couple magnetic flux to the coil typically produces a negative-going voltage and a positive-going voltage, one of which is shorted out to permit the buildup of current in a desired direction in the coil. This current is then interrupted to produce the spark.
The availability of semiconductor devices capable of handling currents at the levels needed by ignition coils has caused increasing use of such semiconductors to replace mechanical breaker points. In the simplest kind of such an electronic module a semiconductor device is placed in series with the current in the coil and is caused to go out of conduction when that current is at or near a maximum value. When the semiconductor device becomes an open circuit, it must be able to withstand the voltage produced by the inductive impulse. The semiconductor device and the other components of the circuit must also withstand whatever reverse voltage is applied to them during the negative half cycle of the voltage developed from the magneto. The semiconductor device and the rest of the module must operate over a range of speeds that is typically at least five or six to one, and may be more. The semiconductor device must carry enough current to develop an adequate spark at the lowest running speed of the engine, and it must handle the higher voltages and associated higher currents produced at the top speed of the engine. The voltage produced by the pickup is typically roughly proportional to engine speed over a considerable range of speeds. It is also desirable in many cases to be able to change the timing of the spark with respect to top dead center (TDC) of the piston. While the engines used in lawn mowers are most often one-cylinder four-cycle engines and those used in chain saws, string trimmers and the like are most often one-cylinder two-cycle engines, the principles discussed here also apply to either type of engine having more than one cylinder.